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The Lorenz Cipher and the World’s First (Secret) Computer

When people think about cryptography during World War II, they typically imagine the Enigma, an enciphering machine built by the Germans. When people think of the first programmable computer, they might recall the ENIAC, developed at the University of Pennsylvania in 1945. However, few people know of the Lorenz cipher machine, or the real first electronic calculator, Colossus.

After the epic failure of their World War I cryptography efforts, Germans entered WWII dedicated to building the ultimate cipher. Germany employed The Lorenz Company to design a teleprinter cipher that would ensure secure radio communication. The Lorenz Company built teleprinters using a strategy created by American Gilbert Vernam in 1918. The teleprinters were based off of the 32-character Baudot Code, as opposed to the standard 26-letter alphabet used in other ciphers, such as the Enigma. The Baudot Code consists of five rows and some number of columns. Each letter is represented by a sequence of 5 symbols, one symbol per row, where each symbol is either a dot or cross. For instance, the letter A might be represented by +..+.+. Random characters were inserted into the cipher text to conceal the message. Plain text was converted to symbol representation and combined with these obscuring letters to produce the cipher text. When the receiver combined the cipher text message with the exact same set of obscuring characters, they can undo the process and reveal the plain text. Vernam had concluded that if the paper used in communication was pre-punched with a completely random set of excess characters, the cipher would be unbreakable. This essential element of the cipher required that both ends of communication use the same pre-punched paper, which was impossible in wartime (Carter).

Unable to implement this security feature of Vernam’s original concept, The Lorenz Company instead designed a machine that would create these excess characters. However, the machine was unable to produce completely random characters; the characters the machine did produce were referred to as “pseudo-random.” Lacking Vernam’s intended stream of random information mixed among the real cipher text, the transmissions did not properly hide the cipher text. Bletchley Park, Britain’s decryption establishment, ultimately capitalized on this seemingly small error—the lack of complete randomness—to decrypt the Lorenz cipher (Sale).

The British first intercepted German communications enciphered with the Lorenz cipher in 1940. John Tiltman, a codebreaker at Bletchley Park, was intrigued by these teleprinter messages, referred to by the codename “Fish.” Tiltman was familiar with the Vernam system and soon recognized that the messages enciphered with the Lorenz cipher utilized the Vernam method. A section of Bletchley Park, known as the Testery, was dedicated to decrypting the German teleprinter messages. The Testery had little success until August 30, 1941, when the Germans made a crucial mistake.

A German operator was transferring a 4,000 character message from one station of the German Army High command to another operator at a different site, possibly Athens to Vienna. The operator used his Lorenz machine to send the receiving operator an indicator that was twelve letters long and used German names. The receiving operator used this information to set up his machine and told the sending operator to go ahead and send the message. The sending operator sent 4,000 characters by hand before the receiving operator replied in German: “didn’t get that – send it again.” Both the sending and receiving operators reset their Lorenz machines to the initial position, a completely prohibited action. The sending operator, once again, keyed in the message by hand.

The crucial mistake occurred in human error. Had the sending operator worked like a machine, the second message and the original message would have been identical, with the exact same keys. However, human error, no doubt due to annoyance and fatigue, caused the sending operator to make mistakes and key a slightly different second message. Had the two messages been exactly the same, the British interceptors would merely have received a useless duplicate of the cipher text with the same actual message and the same pseudo-random excess characters designed to mask the message. Luckily for the British and unfortunatelyfor the Germans, this was not the case. The first message began with SPRUCHNUMMER, German for “message number.” The second time the sending operator keyed in the message, he keyed SPRUCHNR, NR meaning the same thing as NUMMER (Sale). But meaning was not the concern, secrecy was. From this point on, the two messages differed, yet the machine inserted identical pseudo-random characters into the message. This caused the cipher texts to be different. Identical obscuring characters in two different messages with the exact meaning made it easier for the British to identify what these excess pseudo-random characters were (Carter).

British interceptors immediately took interest in these messages, due to the use of the same twelve-letter indicator, and hastily sent them to Bletchley Park for Tiltman to examine. Tiltman used his knowledge and experience to guess SPRUCHNUMMER as the start of the message, allowing him to realize the similarity between the two messages. Tiltman was able to decrypt both messages and used his addition techniques to uncover the excess characters created by the German Lorenz machines. The German errors of re-using a starting position and not keying identical messages allowed Tiltman to crack the cipher and understand the role of the Lorenz machine, without ever seeing a Lorenz machine or knowing how it worked!

Tiltman passed on his discovered sequence of excess characters to Bill Tutte, a recent addition to Bletchley Park. Tutte analyzed the streams of bits in each of the five channels and realized certain patterns when he used a repetition of 41. It became clear that the repetitions could not simply be random, thus indicating that the number 41 held some importance. This discovery allowed Tutte to fully understand the workings of the Lorenz machine (Sale).

In 1942, the Post Office Research Labs at Dollis Hill created a machine similar to the Lorenz using the information discovered by Tutte. Frank Morrell created this machine and called it “Tunny.” This allowed the code breakers at the Testery in Bletchley Park to decipher German messages once they discovered the initial settings of a given message. However, discovering the settings took the Testery four to six weeks, making successfully decrypted information old and useless. To solve this, mathematician Max Newman created an electronic machine that used Tutte’s method to automatically uncover the initial settings for German messages (Carter). This machine was constructed at Dollis Hill and named Heath Robinson. The name “Heath Robinson” was chosen in honor of the cartoonist Heath Robinson who designed incredible machines. Heath Robinson worked well, but Newman later collaborated with engineer Tommy Flowers and others such as mathematician Alan Turing to invent Colossus, the ultimate machine that speedily deciphered the Lorenz Cipher (Sale).

Colossus was fully functioning and breaking the Lorenz Cipher in mere hours, just in time for D-Day (Carter). Using Colossus, British and American forces deciphered messages that allowed Eisenhower and Montgomery, commanders of the allied forces, to receive essential tips on German beliefs and actions. These messages revealed that Hitler had been successfully deceived by the phantom army located in south England and the phantom convoys heading east on the Channel. These phantom armies utilized inflatable tanks and media manipulation to deceive Hitler. Hitler believed the attacks would come from the Pas de Calais and he kept the Panzer divisions in Belgium. This information allowed British and American troops to respond more effectively.

After D-Day, French telephone and teleprinter lines were bombed and the Germans had to communicate with radio, further increasing the amount of intercepted messages and increasing the need for Colossus machines. By the end of the war, there were ten Colossus machines at Bletchley Park. Throughout the war, 63 million characters worth of German messages were successfully decrypted (Sale).

When Japan surrendered after the drop of the atomic bombs, the war quickly came to a close. Eight Colossus machines at Bletchley Park were torn apart and two were sent to Eastcote in North London, then to GCHQ, the Government Communications Headquarters of the UK, in Cheltenham. These two were also torn apart in 1960, and any drawings of Colossus were also burnt then to keep the Colossus a wartime secret. In the 1970’s, information about Colossus re-emerged, and a movement to rebuild a Colossus machine was started by Bletchley Park’s curator, Tony Sale. A new Colossus machine was fully reconstructed in Bletchley Park in 1996. 1996 was a significant year: the 50th anniversary of ENIAC, what the world had believed to be the first electronic digital calculator. This myth arose because Colossus was successfully hidden in secrecy for so many years (Sale).

The Lorenz Cipher and Lorenz machines used by the Germans forced the incredible technological advancement of the true first electronic digital calculator, Colossus. These amazing machines and ciphers played crucial roles in World War II activity, notably impacting the outcome of the war. The Lorenz Cipher and implementing machines advanced cryptography on the side of enciphering and Colossus was revolutionary technology for decrypting. These ciphers and machines have forever left their mark on history and cryptography.

This post is part of a series of essays on the history of cryptography produced by students at Vanderbilt University. The students wrote these essays for an assignment in a first-year writing seminar taught by mathematics instructor Derek Bruff. The essays are shared here, in part, to give the students an authentic and specific audience for their writing. For more information on this cryptography seminar, see the course blog.

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Wonders & Marvels is no longer being updated. This site was created by award-winning author Holly Tucker, who also served as Editor-in-Chief, and ran for over 9 years (2008-2017). We wish to thank the many writers and historians who contributed articles to Wonders & Marvels, including and especially our regular contributors.